1. Field of the Invention
This invention relates generally to digital logic circuits, and, more particularly, to digital logic circuits which receive signals alternating between two different voltage levels.
2. Description of the Related Art
Digital logic circuits typically quantize signal voltage levels to represent two or more logic levels or states. In positive binary logic, a signal voltage level within a voltage range extending downward from a maximum operating voltage typically represents a logic xe2x80x9chighxe2x80x9d level (e.g., a logic xe2x80x981xe2x80x99 state), and a signal voltage level within a voltage range extending upward from a minimum operating voltage typically represents a logic xe2x80x9clowxe2x80x9d level (e.g., a logic xe2x80x980xe2x80x99 state).
Electromagnetic events occurring in and around digital logic circuits produce unwanted xe2x80x9cnoisexe2x80x9d signals. These unwanted noise signals may be coupled into nodes or signal lines of digital logic circuits, causing voltage level variations of logic signals on the nodes or signal lines. For this reason, reliable digital logic circuits incorporate xe2x80x9cnoise marginsxe2x80x9d (NM) for both the logic high level (NMH) and the logic low level (NML). When a noise signal has an amplitude less than NML, and a logic gate receives a signal at an input representing the logic low level and including the noise signal, the noise signal will be attenuated as the received signal passes from the input to an output of the logic gate. Similarly, when a noise signal has an amplitude less than NMH, and the logic gate receives a signal at an input representing the logic high level an including the noise signal,xe2x80x94the noise signal will be attenuated as the received signal passes from the input to an output of the logic gate. As it is equally likely for noise signals to be coupled into signals representing logic low levels and logic high levels, it is desirable that the noise margins NMH and NML be substantially equal.
Power supply conductors used to distribute a xe2x80x9cpositivexe2x80x9d power supply voltage and a reference or xe2x80x9cgroundxe2x80x9d power supply voltage to logic gates of a digital logic circuit have finite electrical resistances and inductances. When the outputs of several of the logic gates change simultaneously, a relatively large switching current xe2x80x9cpulsexe2x80x9d flows through the power supply conductors. The switching current pulse causes voltage drops across the electrical resistances and inductances of the power supply conductors. As a result, the positive power supply voltage level at the logic gates with changing outputs, and at neighboring logic gates, is momentarily reduced, or xe2x80x9cdroops.xe2x80x9d At the same time, the ground power supply voltage level at the logic gates with changing outputs, and at the neighboring logic gates, is momentarily increased, or xe2x80x9cbounces.xe2x80x9d Such power supply droop and ground bounce may generate noise signals within the digital logic circuit. If not attenuated by virtue of noise margins NMH and NML, the noise signals may cause the digital logic circuit to produce incorrect output signals.
Dynamic random access memory (DRAM) devices are commonly used to store data (e.g., within computer systems). Modern synchronous DRAM (SDRAM) devices receive an externally generated clock signal, and use the clock signal to synchronize operations with other devices (e.g., an SDRAM controller). Input signal reception and output signal generation are synchronized with voltage level transitions (i.e., edges) of the clock signal. SDRAM devices include multiple xe2x80x9cbanksxe2x80x9d of memory, and performances of SDRAM devices may be increased by interleaving memory accesses among the multiple memory banks in order to hide required signal line precharge times within the SDRAM devices. In addition, input signals received by more conventional DRAM devices determine the functions performed by the DRAM devices. In contrast, input signals received by SDRAM devices represent commands. The commands may be used to program registers within the SDRAM devices which control operations of the SDRAM devices, thus allowing for programmable operation of SDRAM devices.
A clock signal used to synchronize operations of components of a synchronous digital logic circuit (e.g., including an SDRAM device) alternates periodically between a high voltage level and a low voltage level. The high voltage level may be within a voltage range extending downward from a maximum operating voltage representing a logic high level (e.g., a logic xe2x80x981xe2x80x99 state). Correspondingly, the low voltage level may be within a voltage range extending upward from a minimum operating voltage representing a logic xe2x80x9clowxe2x80x9d level (e.g., a logic xe2x80x980xe2x80x99 state). The operations of the components are typically synchronized to transitions of the clock signal between the high voltage level and the low voltage level (i.e., rising or falling edges of the clock signal). Accordingly, the clock signal must be distributed to the components such that all components xe2x80x9cseexe2x80x9d the edges of the clock signal at substantially the same time. In the manner described above, finite resistances and inductances of conductors used to distribute clock signals (e.g., clock signal conductors and a ground voltage grid or plane), and/or noise signals coupled into the conductors, may reduce the high voltage level of the clock signal andlor increase the low voltage level of the clock signal.
An input buffer of a component receiving a clock signal (e.g., a logic circuit or a clock signal buffer) typically uses a fixed switching point or xe2x80x9ctrip pointxe2x80x9d voltage to produce a xe2x80x9cregenerated clock signal.xe2x80x9d The switching point voltage is typically set to a value mid way between selected xe2x80x9cidealxe2x80x9d high and low voltage levels of the clock signal. The received clock signal voltage is compared to the switching point voltage (e.g., via a comparator). If the clock signal voltage is greater than the switching point voltage, the input buffer may produce the regenerated clock signal within an output voltage range extending downward from the maximum operating voltage and representing the logic high level (e.g., the logic xe2x80x981xe2x80x99 state). On the other hand, if the clock signal voltage is less than the switching point voltage, the input buffer may produce the regenerated clock signal within an output voltage range extending upward from the minimum operating voltage and representing the logic low level (e.g., the logic xe2x80x980xe2x80x99 state).
Practical clock signals transition between the low voltage level and the high voltage level in finite amounts of time (i.e., have finite xe2x80x9crise timesxe2x80x9d), and similarly transition between the high voltage level and the low voltage level in finite lengths of time (i.e., have finite xe2x80x9cfall timesxe2x80x9d). Characterizing such practical clock signals may involve determining a mid voltage level mid way between the high and low voltage levels, and determining xe2x80x9cmid pointsxe2x80x9d of rising and falling edges of the clock signal where the rising and falling edges pass through the mid voltage level. The period of such a practical clock signal may be defined as an amount of time between a mid point of a rising edge of the clock signal and a mid point of the next rising edge of the clock signal. The xe2x80x9cduty cyclexe2x80x9d of a practical clock signal having finite rise and fall times may be defined as a ratio of an amount of time between a mid point of a rising edge of the clock signal to a mid point of the next falling edge to the period of the clock signal.
Where a high voltage level of a clock signal is decreased by an amount when traversing a clock distribution network (i.e., due to conductor electrical characteristics and/or noise signals), and a low voltage level of the clock signal is increased by the same amount, an input buffer employing the above described method for regenerating the clock signal by comparing the clock signal voltage to a fixed switching point voltage may expectedly produce the regenerated clock signal having the same duty cycle as the received clock signal. However, in situations where the high and low voltage levels of the clock signal are changed by different amounts, and when the high and low voltage levels are both increased or both decreased, the regenerated clock signal produced by the input buffer differs from the duty cycle of the received clock signal. As a result of such changes in duty cycle, the components of the digital logic circuit may not xe2x80x9cseexe2x80x9d the edges of the clock signal at substantially the same time. When determining a minimum period of the clock signal, such variations in edge transition times must be accounted for such that the period of the clock signal is sufficient to allow the digital logic circuit to produce correct output signals despite the variations in edge transition times.
It would thus be advantageous to have an input buffer circuit having a variable switching point dependent upon actual high and low voltage levels of a received input signal (e.g., a clock signal), and not a fixed switching point established based upon ideal high and low voltage levels of the input signal. In a synchronous digital logic system using a clock signal to synchronize component operations, such an input buffer would reduce variations in edge transition times of the clock signal received by the components, thereby allowing the period of the clock signal to be reduced, and the performance of the synchronous digital logic system to be increased.
An input buffer is presented for buffering an input signal having a voltage magnitude which alternates between a first voltage level and a second voltage level, where the first and second voltage levels may vary over time. In one embodiment, the input buffer includes a first and second detector circuits, an average generator circuit, and a differential amplifier. The first detector circuit receives the input signal and produces a first signal having a magnitude indicative of the first voltage level. The second detector circuit also receives the input signal, and produces a second signal having a magnitude indicative of the second voltage level.
The average generator circuit receives the first and second signals, and uses the magnitudes of the first and second signals to produce a third signal having a magnitude indicative of a third voltage level substantially mid way between the first voltage level and the second voltage level. The third voltage level defines a variable an automatically adjusted xe2x80x9cswitching pointxe2x80x9d of the input buffer.
The differential amplifier receives the input signal, the third signal, a first power supply voltage, and a second power supply voltage. The differential amplifier amplifies a difference between the voltage magnitude of the input signal and the third voltage level in order to produce an output signal which alternates between the first and second power supply voltages.
An integrated circuit is described including the input buffer coupled between one of a set of input/output pads and circuitry, wherein the circuitry may be synchronous dynamic random access memory (SDRAM) circuitry.
The input signal may alternate between the first voltage level and the second voltage level periodically, and may have a period and a duty cycle. In this situation, the output signal alternates between the first and second power supply voltages periodically, and has a period substantially equal to the period of the input signal and a duty cycle substantially equal to the duty cycle of the input signal. For example, the input signal may be an input clock signal having a period and a duty cycle, and the output signal may be an output clock signal. In this situation, the output clock signal alternates between the first and second power supply voltages periodically, and has a period substantially equal to the period of the input clock signal and a duty cycle substantially equal to the duty cycle of the input clock signal.
As described above, the third voltage level defines a switching point of the input buffer. A first noise margin of the input buffer may exist between the switching point and the first voltage level of the input signal, and a second noise margin of the input buffer may exist between the switching point and the second voltage level of the input signal. In this situation, the switching point of the input buffer varies with changes in the first and second voltage levels of the input signal such that the first and second noise margins are maintained substantially equal.
For example, the first power supply voltage may be denoted xe2x80x9cVDDxe2x80x9d, and the second power supply voltage may be denoted xe2x80x9cVSSxe2x80x9d, where VDD greater than VSS. In addition, the first voltage level may be greater than the second voltage level. Power supply voltage VDD may be greater than or equal to the first voltage level, and power supply voltage VSS may be less than or equal to the second voltage level. In this situation, the first noise margin of the input buffer may be a logic high noise margin NMH, and the second noise margin of the input buffer may be a logic low noise margin NML. The output signal alternates between VDD and VSS, and the switching point of the input buffer varies with changes in the first and second voltage levels of the input signal such that noise margins NMH and NML are maintained substantially equal.
In one embodiment of the input buffer, the first signal is a voltage signal having a voltage magnitude substantially equal to the first voltage level, the second signal is a voltage signal having a voltage magnitude substantially equal to the second voltage level, and the third voltage signal has a voltage magnitude substantially equal to the third voltage level. In this embodiment, the differential amplifier amplifies a difference between the voltage magnitudes of the input signal and the third voltage signal in order to produce the output signal.
For example, as described above, the first power supply voltage may be denoted xe2x80x9cVDDxe2x80x9d, and the second power supply voltage may be denoted xe2x80x9cVSSxe2x80x9d, where VDD greater than VSS. In addition, the first voltage level may be greater than the second voltage level. Power supply voltage VDD may be greater than or equal to the first voltage level, and power supply voltage VSS may be less than or equal to the second voltage level.
The first detector circuit may include a p-channel metal oxide semiconductor (pMOS) transistor and a capacitor, wherein a source terminal of the pMOS transistor receives the input signal, and the capacitor is coupled between a drain terminal of the pMOS transistor and power supply voltage VSS. Electrical current flows through the pMOS transistor and charges the capacitor when the voltage magnitude of the input signal is the first voltage level and a voltage across the capacitor is substantially less that the first voltage level.
The second detector circuit may include a p-channel metal oxide semiconductor (pMOS) transistor and a capacitor, wherein a drain terminal of the pMOS transistor receives the input signal, and the capacitor is coupled between a source terminal of the pMOS transistor and power supply voltage VDD. Electrical current flows through the pMOS transistor and charges the capacitor when the voltage magnitude of the input signal is the second voltage level and a voltage between the source terminal and the second power supply voltage is substantially greater than the second voltage level.
The average generator circuit may include a pair of resistors connected in series between the first and second voltage signals, wherein the third voltage signal is produced at a connection point between the pair of resistors. The pair of resistors may have substantially equal resistance values such that the third voltage signal is substantially mid way between the first voltage level and the second voltage level.
The differential amplifier may include multiple metal oxide semiconductor (MOS) transistors connected together to form a differential network, and an inverter. The differential network may receive the input signal and the third voltage signal, and may amplify the difference between the voltage magnitudes of the input signal and the third voltage signal to produce an intermediate signal. The inverter may receive the intermediate signal at an input terminal and the first and second power supply voltages, and produce the output signal at an output terminal, wherein the output signal alternates between the first and second power supply voltages.
A circuit is described including an input/output pad adapted to receive the input signal, the above input buffer wherein the first and second detector circuits and the differential amplifier receive the input signal via the input/output pad, and circuitry coupled to receive the output signal of the input buffer and configured to perform a function dependent upon the output signal. The input/output pad, the input buffer, and the circuitry may be formed upon and within a single monolithic semiconductor substrate, forming an integrated circuit. As described above, the input signal may be an input clock signal, and the output signal may be an output clock signal. The circuitry may be SDRAM circuitry which uses the output clock signal to synchronize internal operations.
A method for signal buffering embodied within the input buffer includes receiving the above input signal, producing the first signal having a magnitude indicative of the first voltage level of the input signal, and producing the second signal having a magnitude indicative of the second voltage level of the input signal. The first and second signals are used to produce the third signal having a magnitude indicative of a third voltage level substantially mid way between the first voltage level and the second voltage level of the input signal. The difference between the voltage magnitude of the input signal and the third voltage level are amplified to produce an output signal which alternates between the first power supply voltage and the second power supply voltage.